The pending trial of the leading suspect in the 2012 attacks on the U.S. consulate in Benghazi, Libya, is reopening debate not only about the causes of the assaults but also the Obama administration's approach to handling terror suspects.

The administration had hoped it could put to rest both topics—or at least turn down the debate over them. Now they figure to linger as divisive issues for months, a development that could complicate the White House's plan to close the military detention facility at Guantanamo Bay, Cuba,...